Exemplary embodiments of the present invention relate to a digital circuit, and more particularly, to an output driver which outputs an input signal to an external circuit.
In a digital circuit, an output driver transfers a generated signal to other chips. Hence, a final output stage must have higher drivability than any other internal circuit. To this end, an output driver includes a pull-up driver and a pull-down driver. The pull-up driver includes a plurality of PMOS transistors which are coupled in parallel in order to increase a voltage of an output terminal, and the pull-down driver includes a plurality of NMOS transistors which are coupled in parallel in order to lower a voltage of an output terminal.
The pull-up driver refers to a driver which increases current drivability, that is, implements a pull-up driving operation. The pull-down driver refers to a driver which lowers current drivability, that is, implements a pull-down driving operation. The pull-up driving operation means that a specific node of a circuit is coupled to a power supply voltage (VDD) by an active element such as a pull-up transistor. The pull-up driving operation increases an output gain and lowers an output impedance, as compared with a case in which the pull-up driving operation is performed using a resistor. The pull-down driving operation means that a specific node of a circuit is coupled to a ground (VSS) by an active element such as a pull-down transistor. A current gain or an operating speed can be improved, as compared to a case in which the pull-down driving operation is performed using a resistor.
FIG. 1 is a circuit diagram of a conventional output driver.
As shown, the output driver includes a pull-up driving unit 110, a pull-down driving unit 120, a pull-up resistor RU, and a pull-down resistor RD. Specifically, the pull-up driving unit 110 includes a plurality of PMOS transistors coupled in parallel, and the pull-down driving unit 120 includes a plurality of NMOS transistors coupled in parallel. The pull-up resistor RU is coupled between drains of the PMOS transistors and an output terminal, and the pull-down resistor RD is coupled between drains of the NMOS transistors and the output terminal.
FIGS. 2 and 3 are waveform diagrams illustrating an input signal IN inputted to the input terminal INPUT of the output driver and an output signal OUT outputted from the output terminal OUTPUT of the output driver.
When an ideal square wave is inputted as illustrated in FIG. 2, a substantially ideal square wave is also outputted. However, while passing through a package, a printed circuit board (PCB), and so on, the output waveform of the output driver includes harmonic components dispersing outside the circuit or module. In particular, when the output waveform is close to the ideal square wave, as illustrated in FIG. 2, means that many third-order, fifth-order and seventh-order harmonic components are included in the output waveform. This causes an electromagnetic interference (EMI) problem related to the magnitude of the harmonic components. To solve the EMI problem, a slew rate of the output signal OUT is limited as illustrated in FIG. 3 which shows the waveform of the final output terminal. The slew rate may be defined as a voltage change rate with respect to time when the output signal OUT changes from a high level to a low level or changes from a low level to a high level in response to the square wave input signal IN of a system. In a system requiring a high-speed signal transmission, the slew rate is an important factor in determining the characteristics of the system.
Conventionally, as illustrated in FIG. 3, methods of adjusting the slew rate of the input signal have been used to adjust the slew rate of the output signal OUT. That is, in order to solve the EMI problem, the slew rate of the output signal OUT is reduced by reducing the slew rate of the input signal IN. However, as the slew rate of the input signal of the output driver is lowered, the output driver becomes more vulnerable to power noise, causing distortion of the output waveform.